This relates generally to haptic devices, and more particularly to circuits and methods for driving haptic actuators according to a single wire control.
Haptic devices are used in a variety of applications to provide tactile feedback and alerts to users of many consumer devices. For instance, haptic actuators are typically provided in mobile phones to create tactile feedback for users pressing touchscreen softkeys and/or for vibrating the phone to provide silent ringing notification of incoming calls. Also, haptic devices are used to convert audio signals to haptic effects to provide automatic haptics for music, games, movies and the like. Various haptic actuators have been developed, including linear resonant actuators (LRAs) having a spring-loaded mass accelerating and decelerating along a linear axis, as well as eccentric rotating mass (ERM) motors having an eccentric mass affixed to a rotating motor rotor. Haptic actuators are driven by signals causing the actuator to vibrate. Many user devices thus include driver circuits for signaling haptic actuators. The tactile user experience is enhanced by the capability of providing a variety of different distinguishable vibratory signals. Many driver circuits include on-board waveform libraries with a large number of predefined haptic effects or waveforms that can be requested by a host processor through a serial digital interface, such as SPI, I2C, etc., for playback, with a trigger input allowing the waveform playback to be initiated by the host device. Certain haptic driver circuits also include pulse width modulation (PWM) inputs allowing a host device to provide a PWM input signal for tactile actuation, and the driver adjusts the amplitude of a playback waveform according to the PWM input frequency. An enable input is also provided in certain drivers to allow a host device to place the driver circuit in a low power mode.
Maximum acceleration and deceleration of an LRA mass or and ERM motor is important for providing discernible haptic signals to a user. Haptic actuator performance is quantified in terms of start time from zero acceleration to maximum acceleration, and braking or brake time representing the time from maximum acceleration to a rest condition. Some driver circuits for a resonant actuator generate actuator signals at the mechanical resonant frequency, and employ active braking for stopping the actuator quickly. This is done through resonance tracking using detected back EMF representing the velocity of the mass of the haptic actuator to identify and track resonance, and closed loop feedback is used to enhance haptic actuator response time through automatic overdrive and braking control. In the case of an LRA, resonance tracking uses non-trivial configuration by a host device, typically achieved through a digital interface. Many drivers use only open loop control. In these systems, the actuator device is manually characterized, and an appropriate waveform is streamed by a PWM input or a digital interface to achieve fast braking, or the waveforms can be stored in an internal memory of the driver circuit. Power efficiency is also important in battery-powered user devices, and designers of host circuitry need a controllable interface to achieve desired haptic signaling by interfacing with driver circuitry for a variety of end-use applications without unduly consuming battery power.